Reefs From Florida to Caribbean at Risk as Sea Urchin Populations Decline


The long-spined sea urchin may not seem like much, but don’t be fooled by its spiny exterior. These unique creatures are often referred to as the “gardeners of the sea” for their critical role in maintaining healthy coral reefs.

By tending to the algae on their host coral, they prevent it from overrunning and suffocating the reefs. In fact, spotting a long-spined sea urchin in the Florida reef tract is an encouraging sign that the nearby coral is thriving.

However, it wasn’t always this way. In the past, these prickly creatures were seen as more of a hindrance than a help. Divers and marine scientists alike viewed them as a nuisance, and their sharp, poisonous spines made them a hazard to swimmers. Marine scientist Don Levitan remembers visiting the reefs near the U.S. Virgin Islands and being shocked by the sea urchin infestation. “It was so dense it looked like a reef of sea urchins,” Levitan recalls. “You couldn’t even walk into the water.”

These “gardeners of the sea” are a reminder that even the smallest creatures can have a big impact on our planet’s ecosystem.

Thankfully, attitudes have changed over the years, and we now appreciate the critical role that long-spined sea urchins play in maintaining healthy coral reefs. These “gardeners of the sea” are a reminder that even the smallest creatures can have a big impact on our planet’s ecosystem.

Don Levitan, a professor of biological sciences at Florida State University, captures a sea urchin known as Diadema antillarum. Levitan is researching the cause and impact of two massive dieoffs of the Caribbean species. FSU
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In the 1980s, long-spined sea urchins were considered harmful to coral reefs and were viewed as nuisances. But after a mysterious disease wiped out 95% of their population in the Caribbean, including Florida’s reef tract, scientists realized that too few urchins were actually worse for reefs than too many.

After decades of slow recovery, the population of Diadema antillarum, a specific type of sea urchin, has once again drastically declined. A recent study led by marine scientist Don Levitan revealed that a die-off beginning in early 2022 was just as devastating as the previous one, wiping out 98% of the Diadema population. This alarming development is a major setback for coral reefs across the entire region, including Florida.

The Diadema sea urchins, also known as the “billy goats of the sea,” play a crucial role in maintaining the health of coral reefs. These creatures consume macroalgae, preventing it from suffocating coral by depriving it of oxygen. The urchins are prolific grazers, creating algae-free zones or “halos” around coral that allow them to survive. In return, the corals provide shelter for the urchins to hide from predators such as triggerfish or hogfish.

Unfortunately, the coral reefs in Florida are facing multiple threats, including climate change that causes coral bleaching, stony coral tissue loss disease, and pollution from septic tanks and sewage spills that lead to nutrient overload. The decline of the Diadema population due to a recent die-off could have significant consequences for the already struggling reefs in Florida and the wider region.

This 2017 image from a Caribbean coral reef shows a healthy number of diadema sea urchins, which graze algae off coral reefs and allow for better growth.

The critical role played by Diadema in maintaining the health of Florida’s coral reefs cannot be overstated, according to marine scientist Don Levitan. However, the recovery of the species following the first mass die-off in the 1980s was much slower in Florida than in other parts of the Caribbean, and the second die-off has left the natural population almost non-existent. This makes it crucial to focus on conservation efforts to protect the remaining Diadema and their role in preserving the reef ecosystem.

The slow recovery of Diadema in Florida has prompted a collaboration of scientists to work towards reviving the species. However, the first hurdle in this mission is to get the urchins to reproduce and grow up in an aquarium. According to Ken Nedimeyer, technical director of Reef Rescue USA, “Most other urchins are easy to breed and raise, so easy, but Diadema are so hard.” Despite the challenges, recent successes have given hope to the mission. Nonetheless, early attempts to release the raised Diadema into the wild were unsuccessful, with all the urchins disappearing within 24 hours.

“They’re kind of like chocolate-covered peanuts. Everybody likes to eat them on the reef.” stated Nedimeyer.

Scientists have discovered that the biggest culprit of failed releases of captive-raised Diadema urchins into the wild is likely the other neighbors on the reefs. According to Ken Nedimeyer, technical director of Reef Rescue USA, “They’re kind of like chocolate-covered peanuts. Everybody likes to eat them on the reef. Fish like em, crabs like em. They just can’t seem to get past the gauntlet of all the fish mouths trying to eat them.”
Researchers from Reef Rescue transplant elkhorn coral to a new coral reef site in 2019. In the picture, the purple-spined diadema sea urchins appear at healthy levels, a good sign for reef growth.
To address this issue, scientists have tried various methods, such as providing urchins with tiny undersea houses made of concrete or turned-over terracotta pots. In one experiment, fishing lines were even tied to the released urchins to track whether having a hiding spot nearby helped them avoid predators, which it did. However, scientists have also realized that captive-raised urchins behave differently from their wild counterparts and are less equipped to survive. “In the aquarium, they feed them squishy algae, like romaine lettuce for a manatee. We have to teach them to chew,” said Nedimeyer. “In the process of chewing the algae off the rock, they consume some calcium carbonate from the rock that strengthens their spines.”
Despite the challenges, there have been some recent successes in the effort to revive Diadema populations. Researchers at the Florida Aquarium have been raising large batches of the urchins, with the goal of making them “reef competent” and able to survive in the wild. Ken Nedimeyer of Reef Rescue USA expressed optimism about these efforts, stating that “these are one of the most important critters out there. They’re critical for the Caribbean.”
The University of Miami’s Rosenstiel School of Marine, Atmospheric and Earth Science is at the forefront of recent advances in reviving the Diadema population. Inside the school, dozens of baby urchins are being raised in aquarium tanks.
Diego Lirman, an associate professor of marine biology and fisheries division, said that his team has released two batches of lab-spawned Diadema from Patterson’s lab to test sites near Miami Beach, with a third release planned for the summer. Scientists are closely monitoring the released urchins to see if they survive and stay on the reefs or if they’re preyed on by other sea creatures.
Two Diadema antillarum rest on a Caribbean reef. The Caribbean population of this sea urchin has seen two massive dieoffs in the last few decades, which could have lasting impacts on coral reefs.
Scientists have recently shown promising results in their efforts to save coral reefs by reintroducing Diadema urchins to areas where they have disappeared. The team dropped adult Diadema on five reef spots near Key Biscayne and saw a nearly 30% drop in algae cover after three months. After nine months, about 40 of the original 200 urchins transplanted from Port Everglades and Government Cut remained at the Key Biscayne reefs.
This 2017 image from a Caribbean coral reef shows a healthy number of diadema sea urchins, which graze algae off coral reefs and allow for better growth.
The study was published in the journal of the International Coral Reef Society. While more results are needed to confirm the success of this approach, researchers are hopeful that this could be a way to restore coral reefs.
Urchins utilizing critically endangered staghorn corals for shelter one week after being placed on Rainbow Reef. These corals were restored in 2021 during the 100 yards of hope project. While urchins can help corals by eating algae, corals can also help urchins by providing valuable shelter. Dalton Hesley, University of Miami Rescue a Reef Program
The focus for scientists working with Diadema urchins is currently to use them to help struggling reefs rather than to try and resurrect the species. According to Diego Lirman, an associate professor within UM’s marine biology and fisheries division, the eventual goal is to grow the urchins in tanks and release them wherever they’re needed. The aim is to get them to stay and clean the reef to help coral restoration efforts. While dumping millions of competent larvae would be a pipe dream, Lirman remains hopeful that scientists will continue to make progress in their efforts to support the reefs.


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